Fission Versus Fusion
September 17th, 2008
|
| Share |
Fusion as an energy source is something we’ve all heard of as being a kind of “holy grail” of physics and engineering. It’s touted as a potential source of nearly limitless energy, produced from fuel that is so abundant it is inexhaustible and producing absolutely no unwanted waste, no proliferation risk.
But is it really the theoretical “perfect” energy source? Perhaps it could be, but only if a radically different method of creating fusion that is more scalable, simple and stable. But as long as the “dilithium” crystals of Star Trek remain fiction fusion will remain the domain of large magnetic confinement reactors, powerful z-pinch and IEC systems and other similar methods.
Eventually such methods may reach the point of being able to produce more energy than they consume, but if they do, will it really offer any advantages over fission? Perhaps a few, small advantages, but with a number of major drawbacks.
Here’s some information I put together:
| Fission | Fusion | |
| Theory: | Neutrons (uncharged) strike heavy nucleus of atoms causing the nucleus to split releasing energy and more neutrons to continue the process. |
When light element ions are brought together with extreme pressure and energy the Coulomb barrier is overcome and the atoms fuse into a new element, releasing energy in the process. |
| Feasibility: | First demonstrated in 1943. Over 50 years and thousands of examples as a feasible energy production method. | Has never been demonstrated to produce energy, but in theory it should be possible to do so if the process can be scaled up enough while maintaining efficient containment. Unproven. |
| Fuel: | Uranium, a common heavy metal found occasionally in large deposits and common in low concentrations in soil and seawater.Many reactor designs require isotopic enrichment, although this is not universally true. Also thorium, similar to uranium in distribution but about three Thorium and the most common isotopes of uranium are not directly |
Deuterium, an isotope of hydrogen which can be extracted from water, although the process is complex and energy intensive. Tritium, created by neutron bombardment of lithium, especially lithium-6. Helium-3, a decay product of tritium and found in abundance on the moon. Possibly boron, although this remains highly theoretical. |
| Confinement: | Conventional materials: stainless steel, water, graphite, alloys. | Confinement requires that the reacting material is never physically contacted. Accomplished by: extremely powerful opposing magnetic fields, extremely high voltage electrical fields, extreme compression energy, very powerful inductive effects etc. |
| Initiation: | Once the fissile material is brought to critical mass the reaction begins. Using a neutron source, such as Cf-252 or AmBe is standard to assure a smooth startup, but not absolutely necessary. |
Extreme injection of energy to rapidly heat and compress the fuel. Particle beams, high power focused microwaves, RF exciters, the largest capacitor banks in the world, some of the largest lasers in the world in a multi-laser array, or a fission-based explosive. |
| Maintaining the reaction: | The reaction is inherently self-sustaining unless there is a change in the composition such as burning enough fissile material to reduce the critical mass or buildup of “neutron poisons” | Since the reaction requires continuous confinement there must be constant application of energy toward active confinement and control of the reaction. If interrupted the reaction will stop and the reactor may be seriously damaged.
The exception would be pulsed ICF systems which do not actively confine the reaction but instead initiate it repeatedly. |
| Scalability: | Extremely economical at levels of about half a gigawatt or more. Practical and demonstrated numerous times for energy needs of a few megawatts. Possible, although of questionable economics for energy down to a few hundred watts. |
Unknown, however current design efforts favor fusion reactors of extremely high power, because increased scale theoretically will approach “break even” thus focusing on fusion as being used to generate no less than multiple gigawatts from any reactor. |
| Byproducts: | Direct byproducts of fission are lighter elements. Many are radioactive, but most are very short lived. A few have half-life of more than a year and most of those are less radioactive than natural substances and easily disposed of. The few longer lived fission byproducts can be disposed of using Reactors may also product transuranic elements such as plutonium. |
The direct byproduct of fusion is normally helium but may also include mildly radioactive tritium.However, because fusion energy is released entirely through neutrons (fission energy is released though fission fragments) the high neutron flux leaves the reactor vessel and other nearby structures highly radioactive after a few years of use. Use of helium-3 (if even possible) may reduce, but will not |
| Lifetime of Reactor: | Many decades. Reactors are designed for 30+ year lifetimes but may last 60 years or more. | Unknown, as energy producing fusion reactors don’t exist. |
| Necessary Technologies Involved: | Metal forging, cement, steam turbines, generators, plumbing, control systems. | Super conductors, liquid helium compressors, cryonics, specialty alloys, ceramics, magnetic confinement, solid state lasers, gas state lasers, high power RF exciters, super capacitors, diffusion vacuum systems, ion beams, particle accelerators, masers, non-conventional electrical voltages, high pressure insulating gas, plasma dynamics, nuclear resonance, harmonic excitement, pulsed power systems, electrostatics, resonant reactance, neutron-resistant materials, impedance reactors, supercomputer modeling. |
| Track Record: | Decades of safe and economical use. | Decades of research with optimistic promises that it’s “just around the corner” |
| Future: | Molten salt reactors, “deep burn” reactors, integral breeding cycles, ‘sub-critical’ reactors. These technologies are already proven but not fully deployed. | Hopefully it will eventually work at all…someday…maybe. |
This entry was posted on Wednesday, September 17th, 2008 at 2:20 pm and is filed under Enviornment, Good Science, History, Nuclear. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
View blog reactions
September 17th, 2008 at 2:40 pm
Nice data table. While there are certainly some people claiming fusion will be a “holy grail”, I don’t think anyone is actually trying to compare it fission. These energy production methods are not in competition because fission has an industry behind it while fusion is an active research program. Of course, both fields have research arms, but this is like comparing processor chips (industry based) to quantum computing (research based), fields that seek similar goals but are not in competition at the moment. The point is that the money for fusion and fission should be coming from different places and one should not affect the other.
It’s not entirely accurate to say that there are “Decades of failed attempts at “break even”” because break even (while definitely the ultimate goal en route to energy production) has rarely been the task of fusion experiments. There is still a lot of fusion physics that remains to be understood. Experiments are often conducted well away from break even scenarios because there is a wealth of information still to be obtained.
Quote Comment
September 17th, 2008 at 2:53 pm
David Pace said:
Well, it’s true most experiments that were undertaken did not have the direct goal of breaking even, but as an energy source, as early as the 1950’s there was a lot of hope that the Z-pinch system would become an energy source and there were experiments and designs hoping to achieve that end goal. Then when the Tokomak was invented a lot of scientists thought it would be the big breakthrough that would lead to energy. In the 1960’s there were some who thought we were just a few years away from ironing out the issues with the Tokomak and that by the 70’s we’d be riding high on fusion energy.
Then in the 1970’s the statement was made that it was “about 20 years away” and now 30+ years later…
So while I agree that the experiments do have a lot to gain from the standpoint of just learning more about nuclear forces and interactions, the promises of energy from it have failed in general.
Quote Comment
September 17th, 2008 at 3:49 pm
“Has never been demonstrated to produce energy, but in theory it should be possible to do so if the process can be scaled up enough while maintaining efficient containment.
You should add on Earth or something to that effect as it works rather well in stars like the Sun.
Quote Comment
September 17th, 2008 at 3:50 pm
Ya I know – I couldn’t help myself.
Quote Comment
September 17th, 2008 at 4:14 pm
DV82XL said:
What about the H-bomb?
Quote Comment
September 17th, 2008 at 4:40 pm
Gravity makes for a good confinement system. Too bad it doesn’t scale down very well.
Quote Comment
September 17th, 2008 at 6:07 pm
A couple of nitpicks, dilithuim is used in to conduct matter animatter reaction (sorry trekie) also the use of magnetic and electric fieds. The reason for the use magnetic fields is beacuse when matter and antimater contact each other they are converted to energy, so you do not want your antimatter hitting the side of reactor and eating away at it on atom by atom. You could use the electric fields and magentic fields to over the replusion of like charges coming together.
Quote Comment
September 17th, 2008 at 8:35 pm
All kidding aside it’s valid to ask if brute force confinement will ever be a route to controlled terrestrial fusion. As the energies needed to keep the plasma from disassembling increase so do the engineering problems and one is left wondering if a cost effective, widely deployable system will ever be practical and if it is will it be so much better than current nuclear fission systems
The open question is if Muon-catalyzed, Pyroelectric, Migma, Polywell, or Dense plasma focus techniques have any possibility of bearing fruit.
Quote Comment
September 17th, 2008 at 8:38 pm
I could nitpick, but you make a good point that even if we have fusion power with an extension of current systems it will not necessarily have any benefits over nuclear fission and likely will have disadvantages.
It drives me crazy though every time I hear someone say “… until we have fusion energy” as if to assume there will be fusion power in 20 years and we just need a contingency plan to get us by until it is avaliable. You cannot bank on there being scientific discoveries that we have not yet made.
We may have fusion energy tomorrow (in the highly unlikely event that someone comes up with a breakthrough method of creating fusion) or we may never have fusion energy, or we may have it in ten years or one hundred years, or we may have it in ten years and then find it is not worth the trouble.
At the moment our best bet seems to be building the systems we have now only making them bigger and refining some of the design issues. I’m not very hopeful that will lead to an economical and useful fusion energy system. Lets not forget that step one is breaking even in the laboratory on a prototype system and step two is turning that into a mass deployment commercial system, and there’s a veritable canyon between those two steps. We’ve not even really reached step one yet. (we may never or we may in the relatively near future. I don’t know.)
Quote Comment
September 17th, 2008 at 9:00 pm
An Actual Scientist said:
You have hit the nail on the head Actual; this is a huge part of the problem in getting everyone that doesn’t buy into the Green agenda, but believes we have to find new energy, onside for new nuclear builds.
Quote Comment
September 18th, 2008 at 5:18 am
A few spelling errors: column barrier (first box of fusion); transonic (byproducts of fission; shouldn’t this be transuranic?). But it’s nice to get a good comparison of both.
Quote Comment
September 18th, 2008 at 5:44 am
Very nice analysis. Fusion power, as we currently know it, has significant disadvantages as compared to Fission power, even if it “breaks even”.
I think none of our energy plans should incorporate the idea of fusion, or dyson spheres or any such figments from science fiction. True, we will see marvelous things in the future. But the plans that we make for present (or for the next 200 years) should not be dependent on these wish lists.
Having said that, we humans have a very deep problem with understanding exponential growth. If you asked someone 100 years ago to make an energy plan, he would have considered only coal, not even oil. Now just after 100 years, we are close to extinguishing oil reserves and about to tap into nuclear fission power for real.
It is quite possible that something drastic will happen in the next 100 years, we cannot reach the summits of the exponential curve and imagine how it feels like over there. The best we can do is to plan for the present. Future will be doubly exciting. Who knows, we might discover better ways of producing power by tapping directly into mass energy equivalence.. ? Or probably there is an equivalence even more fundamental in physics that will be discovered ??
Quote Comment
September 18th, 2008 at 7:33 am
While I agree that Fission is a provable working technology and it should be looked at right now as part of our solution for the upcoming ‘Oh crap we really f**ked up’ situation and also agree that ‘Ooo we’ll get Fusion working any day now’ is not an actual rational energy policy, I do think we should fund more Fusion research, especially some of the less funded less large scale ideas. I have to say I’m a fan of Bussards Pollywell research.
I do wonder is the ITER thing is funded because it’s a nice way to spend lots of money without actually upsetting the current apple cart. But I’m old an cynical like that. ‘Cheap’ nuclear fusion would upset too many entrenched interested, expensive fusion (or at least comparably priced to fission) is easier to handle.
Quote Comment
September 18th, 2008 at 12:07 pm
David Pace said:
Yeah, I think you’re right. I changed that. It was a bit deceptive.
Vjatcheslav said:
Noted and corrected
Quote Comment
September 18th, 2008 at 4:11 pm
David Pace said:
Question: Would you say most fusion research, fusion reactors/facilities and funding exists to the end goal of creating a viable energy source or simply as a pure science kind of pursuit to better understand fusion and plasma physics in general?
I know with ITER all I hear is how they really want to get to the point of putting out energy and demonstrate it as a power source. It sounds like at least that project is more of an energy thing than just understanding it better. Or maybe they just say that to get funding?
Quote Comment
September 19th, 2008 at 3:53 am
One of my nuclear engineering professors thought that fusion was a bust barring some kind of unforeseen technological breakthrough. He argued that the fusion process releases high-energy neutrons and would have to be heavily shielded. Neutron activation would mean the whole mess would be a disposal nightmare at the end-of-life. The neutron embrittlement of metals was another issue that he thought could be a serious problem.
Quote Comment
September 19th, 2008 at 11:16 am
Nuclear fusion (should it be workable) potentially has a pivotally important role in the development of space travel. The He-3/D reaction yields the highest energy output per mass of any substance found in nature (matter/antimatter reactions have much higher energy per mass, but antimatter mines are a bit on the rare side), and therefore yields the highest specific impulse if used as rocket fuel. A fusion rocket using He-3 could be designed for a velocity of ~10% lightspeed. Fission has the advantage of power density, but that is not an issue in this case. A helium-3 fusion rocket could reach nearby stars within decades of launch. Fission-based rockets can’t match that performance.
Quote Comment
February 14th, 2009 at 5:03 pm
If we manage to get fusion working as a large-scale power source, which applications would find it especially useful, and which would be better off sticking with fission? I imagine that if fusion is restricted to huge reactors generating tens of gigawatts each, it could be problematic for grid electricity (and therefore perhaps better used for synthetic fuel production).
Also, what do people think of hybrid reactors, which use an energy-subcritical fusion reactor to provide additional neutrons for a fission breeder reactor? This exploits the fact that fission is energy-rich but neutron-poor, while fusion is energy-poor but neutron-rich.
Quote Comment